(1) Field of the Invention
The present invention relates to a process, method and apparatus, with particular reference to the manufacture of fans and turbines (for dynamic interaction with fluid flows) and guide vanes (for static interaction with fluid flows), especially for use in turbomolecular pumps and in gas turbine engines.
(2) Description of Prior Art
Gas turbine engines are a widely used form of internal combustion engine and are in many senses more efficient than reciprocating engines operating on the two- or four-stroke principle. In particular, for a given size, the gas turbine can give a higher power output.
Much gas turbine development has concentrated on relatively large power plants for large power outputs, exemplified by turbojet and turbofan engines for aeronautical use. In scaling down the size of such engines, engineering problems are met as dimensions decrease. Some of these problems are to do with the difficulties of further miniaturisation at a manufacturing level, and some to do with the behaviours of gas flows in relatively small spaces.
The present invention addresses aspects of gas turbine engine construction that are particularly applicable to very small gas turbines. Such engines are exemplified by a particular embodiment given as an example herein, where the invention is applied to the manufacture of components for a turbojet engine with an overall diameter of about 10 cm and length of about 32 cm, developing 110N (27 lb) of thrust at an engine speed of 90,000 rpm.
In the general form of a gas turbine, a rotary compressor raises the pressure of intake air, at least some of the air is passed to a combustion chamber or chambers where fuel is burned, exhaust gases drive a turbine as they pass to an exhaust nozzle, and the turbine drives the compressor by an engine shaft.
Gas flow through the engine is highly influenced by a number of static and moving surfaces, typically including fan blades on the successive rotating discs of a multistage axial compressor, stator vanes between each pair of adjacent discs and after the last disc, static nozzle guide vanes between the combustion chamber and turbine, and the rotating turbine blades.
The vanes and blades are likely to be of aerofoil cross section. They are arranged in a substantially radial alignment (as in the spokes of a wheel). The velocity of the axial gas flow through the engine changes from one axial location to the next. At any given axial location in the compressor or turbine section of the engine, the axial gas velocity is intended to be substantially uniform across the diameter of the engine. Since, in the case of rotary fans and turbines, the outer tips of the blades are moving much faster than their inner roots, and because of centrifugal effects, the blades on rotary components are designed to compensate. The blade section changes with radial distance from the centre. Typically, the blades appear twisted along their length, with the least stagger angle at the root and the greatest stagger angle at the tip.
These vanes and blades are conventionally made by casting or machining each blade out of a suitable strong and heat resistant metal alloy. After each individual blade is made, it must be fixed into position on a suitable compressor or turbine hub. This process requires suitable connecting means for making connections between the adjacent parts. In a smaller engine, more precision is required in such means, because any misalignments will be proportionally more significant. The connecting means should be strong, but light in weight. Bonding techniques are used in some cases to attach cast turbine blades to a central rotary disc. In the case of rotary blade discs, a balancing operation will be necessary after assembly.
In the case of a very small engine, a disc might be cast with integral blades, but there are serious problems due to the proximity of adjacent blades, and their twisted forms. While the casting operation as such is technically feasible, the construction of the pattern is exceedingly difficult, and might almost be considered impossible, to construct and use a jig to achieve correct and uniform blade angles, radii and spacings and the like.
In the example of a miniature jet engine given above, the compressor and turbine discs, including blades, may be about 8 cm in diameter, blade tip to blade tip, carrying between 24 and 40 closely spaced thin aerofoil blades, whose chord lengths and stagger angles vary over the lengths of the blades to result in substantial overlap between neighboring blades both axially and radially. The problem of constructing such bladed discs has for a long time been intractable.
In accordance with the disclosure of our patent application PCT/GB2004/000774, published as WO2004/076111, the problem of accurately forming a closely spaced radial array of overlapping shaped blades or vanes, particularly for use in an axial compressor or turbine in a gas turbine engine, and especially in a miniature gas turbine engine, is addressed by removing material from a solid blank to leave the blades or vanes upstanding as the residual material of the blank, removal being effected by a change of state of the material induced by proximity to an advancing tool, the tool being in the form of a shaped wire, and the tool turning as it advances, whereby to generate a surface of a said shaped blade or vane.
We have now devised improvements in the invention disclosed in our said application WO2004/076111.
Our previous application disclosed a tool for removing material from a workpiece blank, such tool being in the form of a shaped wire. In the case where electro-discharge machining, also known as spark erosion, is the method of removing the material, our previous application discloses the use of a wire tool electrode, and specifically a shaped wire tool electrode.